Low pressure mercury discharge lamps, such as fluorescent lamps, rely upon mercury vapor for producing a discharge. The lumen output of such lamps is a function of mercury vapor pressure, which, in turn, generally depends on the cold spot temperature of the lamp envelope. It is well known to those skilled in the art that the cold spot temperature for most efficient lamp operation is approximately 40.degree. C., which produces a mercury vapor pressure of approximately 4-6.times..sup.-3 torr inside the lamp envelope. Due to high lamp loading and high ambient temperatures, the lamp envelope temperature and mercury vapor pressure are frequently above the optimum value. For nonoptimum mercury pressures, the light output of the lamp may decrease significantly. One known method for regulating mercury vapor pressure at high ambient temperatures or high wall temperatures is to use an amalgam. In addition to reducing the mercury vapor pressure at high temperatures, the amalgam produces a stable mercury pressure over a broader temperature range. The use of an amalgam in low pressure mercury vapor lamps is disclosed, for example, in U.S. Pat. No. 3,742,278 issued Jun. 26, 1973 to Shindelman et al.
The stabilization of mercury vapor pressure over a broad temperature range provided by an amalgam is frequently inadequate to encompass many typical operating conditions. For example, a lamp with an amalgam that is designed to operate efficiently under adverse conditions, such as in unventilated enclosed fixtures, may not operate efficiently at normal ambient temperatures, and the light output may be reduced by 20% or more. A result of the limited temperature range of amalgams is that different lamps with different amalgams are required for different applications, depending on the expected operating temperature.
The light output of low pressure mercury discharge lamps, such as compact fluorescent lamps, is sensitive to lamp orientation. It has been demonstrated in one instance, for example, that a 26 watt double twin tube amalgam-containing compact fluorescent lamp manufactured by GTE Products Corporation, when operated in a base up mode, has a more uniform light output over a broader ambient temperature range than the same lamp operated in a horizontal mode. It has been determined, for instance in this case, that the amalgam located in the base of the lamp has an operating temperature 40.degree. C. higher in a base up orientation due to heat convection. This higher temperature coincides with a more ideal mercury vapor pressure above the amalgam than when the lamp is operated in a horizontal orientation. Thus, the amalgam operates at a more optimum temperature in the base up mode. Since the light output for both amalgam and non-amalgam lamps is sensitive to lamp orientation, the user must specify the lamp orientation to obtain a lamp having the maximum light output. Alternatively, the lamp is used in a nonoptimum orientation, and the light output is reduced.
Since fluorescent lamps have a limited operating temperature range and are sensitive to orientation, it has been necessary to design different lamps for different applications. This is inefficient in terms of manufacturing, inventory and sales, and adds to the cost of fluorescent lamps.
Techniques for controlling mercury vapor pressure in low pressure mercury discharge lamps have been disclosed in the prior art. A sleeve-type heater for an aperture fluorescent lamp used in a photocopier is disclosed in U.S. Pat. No. 4,827,313 issued May 2, 1989 to Corona. A film heater for a discharge lamp used for back lighting of crystal displays is disclosed in U.S. Pat. No. 4,931,685 issued Jun. 5, 1990 to Dobashi et al. A compact fluorescent lamp wherein mercury vapor pressure is controlled by controlling the flow of cooling air around the lamp is disclosed in U.S. Pat. No. 4,694,215 issued Sep. 15, 1987 to Hofman. All of the known prior art techniques for controlling mercury vapor pressure in low pressure mercury discharge lamps have had one or more disadvantages, including blockage of light output by film or sleeve type heaters and complexities in circuit designs, which add substantial costs.
It is a general object of the present invention to provide improved low pressure mercury discharge lamps.
It is another object of the present invention to provide low pressure mercury discharge lamps wherein the light output level is substantially constant over a broader range of ambient temperatures.
It is a further object of the present invention to provide low pressure mercury discharge lamps wherein the light output level is substantially constant for different lamp orientations.
It is a yet another object of the present invention to provide low pressure mercury discharge lamps wherein mercury vapor pressure is thermostatically controlled, with negligible blockage of light emitted from the lamp.
It is still another object of the present invention to provide low pressure mercury discharge lamps wherein the temperature of an amalgam located in an exhaust tubulation is thermostatically controlled.